DESCRIPTION (Verbatim from the Applicant's Abstract): In myocardial ischemia, arrhythmias due to excessive norepinephrine (NE) release are responsible for high morbidity and mortality. Hence, reduction of NE release from cardiac sympathetic nerve endings (SNE) is a critical goal. We will study the mechanisms by which endogenous modulators regulate cardiac sympathetic neurotransmission under normal and ischemic conditions. In Specific Aim 1, we will investigate the transductional mechanisms by which histamine H3-receptors (H3R) attenuate Ca2+-dependent NE exocytosis and carrier-mediated NE release, associated with short-term and protracted ischemia/reperfusion (I/R), respectively. We will examine the cardioprotective effects of H3R in cell lines transfected with cloned H3R, and in SNE isolated from guinea-pig and human hearts which express H3R. For NE exocytosis and short-term I/R we will assess the role of N-type Ca2+-channel inhibition, and decreased activities of phosphoinositide and cAMP pathways. For carrier-mediated NE release and protracted I/R, we will investigate the coupling of H3R activation to Na+/H+ exchanger inhibition and identify the transductional mechanisms involved. In Specific Aim 2, we will expand on our recent findings that ATP, co-released with NE from cardiac SNE, enhances NE release, and that SNE are endowed with ectonucleotidase activity. Thus, we will examine the ability of ecto-nucleotidases to control the effects of ATP. We will also investigate whether ecto-nucleotidase activity is enhanced by SNE depolarization, how it affects ATP-induced modulation of NE release, and whether it decreases during ischemia, leading to enhanced NE release. In Specific Aim 3, we will investigate the control of NE release by vasopressin (AVP) and oxytocin (OT), peptides we have recently found to be present in cardiac SNE, to be released during L'R in isolated guinea-pig hearts, and to elicit NE release when added to cardiac SNE. We will investigate the AVP and OT receptor subtypes and transductional mechanisms involved in AVP- and OT-induced enhancement of NE release under normal and ischemic conditions. These studies will elucidate the mechanisms for endogenous control of NE release. Further, since reduction of NE release is protective in myocardial ischemia, identification and characterization of the factors controlling NE release will provide the basis for the development of novel therapeutic strategies in cardiovascular diseases.